1. Field of the Invention
The present invention generally relates to a light emitting diode structure and method of fabricating the same, and more particularly, to a light emitting diode structure having a strain-reducing seed layer for reducing the dislocation density thereof and a method of fabricating the same.
2. Description of Related Art
Light emitting diode is a semiconductor device used in a light emitting device. Because of low power consumption, low contamination, long working life, and rapid responding speed, light emitting diode is useful in many areas such as traffic bulletin boards, outdoor display panels and the back light of displays. Therefore, light emitting diodes have become one of the most important products in the optoelectronic industry.
FIG. 1 is a schematic cross-sectional view of a convention light emitting diode. As shown in FIG. 1, a conventional light emitting diode 100 includes a substrate 110, a buffer layer 120, a first type doped semiconductor layer 130, a light emitting layer 140, a second type doped semiconductor layer 150, a first electrode 160 and a second electrode 170. The buffer layer 120 is disposed on the substrate 110. The first type doped semiconductor layer 130 is disposed on the buffer layer 120. The light emitting layer 140 is disposed on a portion of the area of the first type doped semiconductor layer 130. The second type doped semiconductor layer 150 is disposed on the light emitting layer 130. The first electrode 160 is disposed on the first type doped semiconductor layer 130 not covered by the light emitting layer 140, and the second electrode 170 is disposed on the second type doped semiconductor layer 150.
As shown in FIG. 1, there is a large difference between the lattice constant of the first type doped semiconductor layer 130 and the lattice constant of the substrate 110. Therefore, the light emitting diode 100 normally has a buffer layer 120 disposed between the first type doped semiconductor layer 130 and the substrate 110 for reducing lattice mismatch due to the difference in their lattice constants. In other words, the buffer layer 120 can be used to improve the epitaxial quality of the first type doped semiconductor layer 130, the light emitting layer 140 and the second type doped semiconductor layer 150, and hence prevent the efficiency of the light emitting diode 100 from being affected.
In the conventional light emitting diode 100, material having a lower epitaxial temperature such as aluminum-gallium nitride (AlxGa1-xN) or indium-gallium nitride (InxGa1-xN) is normally used as the material of the buffer layer 120. Next, a gallium nitride (GaN) layer is formed on the buffer layer 120 to serve as the first type doped semiconductor layer 130. However, the gallium nitride layer grown on the buffer layer 120 may have a dislocation density in excess of 5×108 cm−2. The high dislocation density can have some adverse effects on the property and performance of the light emitting diode 100.
To resolve the problem of having a high dislocation density, U.S. Pat. No. 6,847,046 has disclosed a method of using a super lattice structure formed by combining a silicon nitride (SiNx) compound and an aluminum-indium-gallium nitride (AlInGaN) to serve as the buffer layer of the light emitting diode. However, the clusters formed by the silicon nitride compound have a small distance of separation, only in the nanometer scale. Therefore, when aluminum-indium-gallium nitride is subsequently grown in an epitaxial process on the silicon nitride compound, the dislocation density can be lowered to at most 108 cm−2 because the clusters formed by the silicon nitride compound are separate only on the nanometer scale. Consequently, it is very difficult to improve the quality of the light emitting diode any further.